Liquid cooled attenuator

ABSTRACT

A traveling-wave tube coupled interaction cavity slow-wave structure with longitudinally extending attenuating transmission lines having a distributed loss in a particular desired range. The degree of loss is varied to achieve desired electronic efficiency and stability. The variance is achieved by physically and electromagnetically coupling one or more transmission lines directly to the coupling holes between interaction cavities via irises places at various angles to the normal plane, by varying the width of the irises, and also by varying the placement of the transmission lines with respect to the interaction cavities.

United States Patent Winslow et al.

LIQUID COOLED ATTENUATOR Inventors: Lester M. Winslow, Alexandria, Va.;

Hazel E. Brown, Washington, DC.

The United States of America as represented by the Secretary of theNavy, Washington, DC.

Filed: Oct. 24, 1973 Appl. No.1 409,204

Assignee:

US. Cl. 315/35; 315/36; 315/393; 333/31 A Int. Cl. H01 j 25/34 Field ofSearch 315/35, 3.6, 39.3; 333/31 A References Cited UNITED STATESPATENTS 6/1967 Winslow 315/35 11/1967 Hant 315/35 7/1 69 Cerko 315/35 1June 10, 1975 3,602,766 8/1971 Grant ..3l5/3.6 3,771,010 11/1973 Winslow..315/3.5

Primary ExaminerSaxfield Chatmon, Jr. Attorney, Agent, or FirmR. S.Sciascia; Arthur L. Branning [5 7] ABSTRACT A traveling-wave tubecoupled interaction cavity slowwave structure with longitudinallyextending attenuating transmission lines having a distributed loss in aparticular desired range. The degree of loss is varied to achievedesired electronic efficiency and stability. The variance is achieved byphysically and electromagnetically coupling one or more transmissionlines directly to the coupling holes between interaction cavities viairises places at various angles to the normal plane, by varying thewidth of the irises, and also by varying the placement of thetransmission lines with respect to the interaction cavities.

10 Claims, 6 Drawing Figures PATENTEDJUH 10 1915 :2, 889,149

SHEET 2 LIQUID COOLED ATTENUATOR BACKGROUND OF THE INVENTION Thisinvention relates generally to microwave devices. and more particularlyto traveling-wave tubes having means for substantially eliminatingoscillation at frequencies at the edges of the frequency passband of thetube, as well as for providing a carefully shaped gain vs. frequencycharacteristic.

In travelling-wave tubes a stream of electrons is caused to interactwith a propagating electromagnetic wave in a manner which amplifies theelectromagnetic energy. In order to achieve such interaction, theelectromagnetic wave is propagated along a slow-wave structure. Thisstructure may be in the form of a conductive helix wound about the pathof the electron stream or a folded waveguide type of structure in whicha waveguide is effectively wound back and forth across the path of theelectrons. The slow-wave structure provides a path of propagation forthe electromagnetic wave which is considerably longer than the axiallength of the structure. Through resort to this difference in pathlengths, the traveling-wave may be made to effectively propagate atnearly the velocity of the electron stream. The interactions between theelectrons in the stream and the traveling wave cause velocitymodulations and bunching of the electrons in the stream. The net resultwill then be a transfer of energy from the electron beam to the wavetraveling along the slowwave structure.

The present invention is primarily, although not necessarily, concernedwith traveling-wave tubes utilizing slow-wave structures of the coupledcavity, or interconnected cell, type. In this type of slow-wavestructure a series of interaction cavities, or cells, are disposedadjacent to each other sequentially along the axis of the tube.

' In operation, electron stream passes through each interaction cell.Electromagnetic magnetic coupling is provided between each cell and theelectron stream. Each interaction cell is also coupled to an adjacentcell by means ofa coupling hole at the end wall defining the cell.Generally, the coupling holes betwen adjacent cells are alternatelydisposed on opposite sides of the axis of the tube, although variousother arrangements for staggering the coupling holes are possible andhave been employed. When the coupling holes are so arranged, a foldedwaveguide type of energy propagation results, with the traveling-waveenergy traversing the length of the tube by entering each interactioncell from one side, crossing the electron stream and then leaving thecell from the other side, thus traveling a sinuous, or serpentine,extended path.

One of the problems encountered in traveling-wave tubes of the coupledcavity variety, and especially high power tubes of this type, is atendency for the tube to oscillate at frequencies near the edges of thetube passband. This problem arises from the fact that for widebandoperation the phase velocity of the slow-wave circuit wave and thevelocity of the electron beam should be essentially synchronized over aslarge a range of frequencies as possible; hence, these velocities arealso close to synchronism near the upper and lower cutoff frequencies ofthe tube. Since the interaction impedance is high and the circuitimpedance varies rapidly, usually creating high reflections, at and inthe vicinity of the cutoff frequencies, the loop gain for the tube, or

even for a section of the tube, may be sufficiently large foroscillations to start.

One technique which has been used to solve this oscillation probleminvolves coupling to the slow-wave structure interaction cells speciallydesigned cavities which are sharply resonant at a frequency in thevicinity ofa cutoff frequency of the slow-wave structure and providinglossy ceramic buttons in these special cavities in order to attenuateenergy at the resonant frequency of the cavity. While this technique isuseful for attenuating energy at those frequencies where the tube ismost likely to oscillate without substantially affecting energy atfrequencies throughout the remainder of the tube passband, a minimumreflection coefficient is not provided. A low reflection coefficient ishighly desirable in preventing large fluctuations in gain as a functionof frequency at the low frequency end of the tube passband.

Another technique has been developed wherein the resonant cavities andceramic buttons are replaced by a longitudinally extending attenuatingtransmission line. This line typically may be a cylindrical or tubularlossy ceramic rod disposed coaxially about an electrically conductiverod.

Coupling irises in the side walls of at least certain ones of theslow-wave structure interaction cavities provide electromagneticcoupling between the slowwave structure and the transmission line. Thistechnique is disclosed in US. Pat. No. 3,771,010, filed Nov. 22, 1972 byWinslow.

SUMMARY OF THE INVENTION The present invention includes means in atravelingwave tube for providing a stream of electrons along apredetermined path and a slow-wave structure having a plurality ofintercoupled interaction cavities disposed sequentially along and aboutthe electron stream path for propagating electromagnetic wave energy insuch manner that it interacts with the stream of electrons. Anattenuating transmission line is disposed proximate to and externally ofthe electron stream path, with the longitudinal axis of the lossyelement being parallel to the electron stream path. Coupling irises aredisposed so that at least certain ones of them couple the transmissionlines directly into one of the coupling holes which lie between adjacentinteraction cavities. The coupling irises may be angled into thecoupling holes from both transmission lines to each coupling hole or thetransmission lines may be coupled alternatively into the coupling holesby alternate coupling irises. The transmission lines may also extendlaterally into the interaction cavities, be tangent thereto, or bewholely outside the interaction cavitiessCombinations of the above maybe used to give the desired degree of loss in a particular range.

Accordingly, it is an object to the present invention to provide atraveling-wave tube in which any tendency for the tube to oscillate inthe vicinity of the edges of the tube frequency passband insubstantially eliminated, and at the same time, in which a minimumreflection coefficient is provided to minimize small signal gainvariations at the low end of the frequency passband of the tube.

It is further object of the present invention to provide a coupledcavity traveling-wave tube having a readily controllable and carefullyshaped gain vs. frequency characteristic.

It is still further object of the present invention to provide means forboth suppressing oscillations and shaping the gain vs. frequencycharacteristic of a high power traveling-wave tube of the coupled cavitytype, and which means is simpler in design and requires fewer parts thanschemes heretofore employed.

Another object of the present invention is to provide means to vary thedegree of loss in a simple manner to achieve high electronic efficiencyover the bandwidth of the tube.

Other objects, advantages and novel features of the invention willbecome apparant from the following detailed description of the inventionwhen considered in conjunction with the accompanying drawings wherein:

'BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an overall view partly inlongitudinal section and partly broken away of a traveling-wave tubeconstructed in accordance with the present invention.

FIG. 2 is crosssectional view thken along line 2-2 of FIG. 1 showing theprior art arrangement.

FIGS. 3A and 3B show cross-sectional views taken along line 2-2 of FIG.1 showing one embodiment of the present invention.

FIGS. 4A and 4B show a cross-sectional view taken along lines 2-2 ofFIG. 1 showing another embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referrring now to FIG. 1,numeral designates generally a traveling-wave tube which includes anarragnement 12 of magnets, pole pieces and spacer elements, which aredescribed in detail in US. Pat. No. 3,324,338, to Winslow. At this pointit should suffice to state that the spacer elements and interiorportions of the pole pieces function as a slow-wave structure, while themagnets and pole pieces constitute a periodic focusing device for theelectron beam traversing the length of the slow-wave structure.

Coupled to the input end of the arrangement 12 is an input waveguidetransducer 14 which includes an impedance step transformer 16. A flange18 is provided for coupling the assembled traveling-wave tube 10 to anexternal waveguide or other microwave transmission line (not shown). Theconstruction of the flange 18 may include a microwave window (not shown)transparent to microwave energy but capable of maintaining a vacuumwithin the traveling-wave tube 10. At the output end of the arrangement12 an output transducer 20 is provided which is substantially similar tothe input transducer 14 and which includes an impedance step transformer22 and a coupling flange 24, which elements are similar to the elements16 and 18, respectively of the input transducer 14. For vacuum pumpingor outgassing the traveling-wave tube 10 during manufacture, adouble-ended pumping tube 26 is connected to both the input and outputwaveguide transducers 14 and 20.

An electron gun 28 is disposed at one end of the traveling-wave tube 10which, although illustrated as the imput end in FIG. 1, mayalternatively be the output end if a backward wave device is desired.The electron gun 28 functions to project a stream of electrons along theaxis of the tube 10 and may be of any conventional construction wellknown in the art. For details as to the construction of the gun 28reference is made to US.

Pat. No. 2,985,791, entitled, Periodically Focused SeveredTraveling-Wave Tube," issued May 23, 1961 to D. J. Bates et al, and toUS. Pat. No. 2,936,393, entitled, Low Noise Traveling-Wave Tube, issuedMay 10, I960, to M. R. Currie et al.

At the output end of the traveling-wave tube 10 there is provided acooled collector structure 30 for collecting the electrons in thestream. The collector is conventional and may be of any form well knownin the art. For details as to the construction of the collector,reference is made to the aforesaid US. Pat. No. 2,985.79] and to US.Pat. No. 2,860,277, entitled Traveling- Wave Tube Collector Electrode,issued Nov. 1 l. 1958, to A. H. Iversen.

Referring to FIG. 2, a view along 2-2 of FIG. 1 shows an essentiallyannular dish-shaped focusing magnet 32 interposed longitudinally alongthe tube between two ferromagnetic pole pieces 34, only one of which isillustrated. The ferromagnetic pole pieces 34 extend radially inwardlyto the magnets 32 to approximately the perimeter of the region adaptedto contain the axial electron stream. The individual pole pieces areconstructed in such a manner that a short drift tube, or ferrule, 36 isprovided at the inner extremity of each pole piece. The drift tube 36 isin the form ofa cylindrical extension, or lip, protruding axially alongthe path of the electron stream from both surfaces of pole piece 34 i.e.in both directions normal to the plane of the pole piece 34. The drifttubes 36 provided with central and axially aligned apertures 38 toprovide a passage for the flow of the electron beam. Adjacent ones ofthe drift tubes 36 are separated by a gap which functions as a magneticgap to provide a focusing lens for the electron beam and also as aninteraction gap in which energy exchange between the electron beam andtraveling-wave energy traversing the slow-wave structure occurs.

Disposed radially within each of the magnets 32 a slow-wave circuitspacer element 42 of a conductive nonmagnetic material such as copper.Each spacer element 42 has an annular portion of an outer diameteressentially equal to the inner diameter of the magnetic 32 and a pair ofoppositely disposed ear portions 43 and 44 projecting outwardly from theannular portion. Each spacer element also defines a central cylindricalaperture 45 to provide space for the microwave inter action cell, orcavity which is defined by the inner lat-.

eral surface of the spacer 42 and the walls of the two adjacent polepieces 34 projecting inwardly of the spacer element 42. The innerdiameter of the spacer 42 determines the radial extent of theinteraction cell while the axial length of the spacer 42 determines theaxial length of the cell.

For interconnecting adjacent interaction cavities an off-center couplinghole 48 is provided through each of the pole pieces 34 to permit thetransfer of electromagnetic wave energy from cell to cell. As isillustrated, the coupling holes 48 may be substantially kidney-shapedand may be alternately disposed apart with respect to the drift tubes36. It should be pointed out, however, that the coupling holes 48 may beof other shapes and may be staggered in various other arrangements. Inany event, it will be apparent that the spacer elements 42 and theportions of the pole pieces 34 projecting inwardly of the spacers 42 notonly form an envelope for the tube, but also constitute a slow-wavestructure for propagating travelingwave energy in a serpentine pathalong the axially traveling electron stream so as to support energyexchange between the electrons of the stream and the traveling-wave.

The axial length of the magnets 32, hence that of the spacers 42 isequal to the spacing between adjacent pole pieces 34, and the radialextent of the magnets 32 is approximately equal to or, as shown,slightly greater than that of the pole pieces 34. To provide focusinglenses in the gaps, the magnets'32 are stacked with alternating polarityalong the axis of the tube, thus causing a reversal of the magneticfield at each magnetic lens and thereby providing a periodic focusingdevice. It should be pointed out, however, that although the lengths ofthe spacers 42 may be substantially constant, they may also be variedslightly with respect to each other so that the effective axial lengthof the cavities is varied as a function of distance along the tube toensure that the desired interaction between the electron stream and thetraveling waves will continue to a maximum degree even though theelectrons are decelerated toward the collector end of the tube.

To provide the desired attenuation, lossy transmission lines 56 and 60are utilized. The first lossy transmission line 56 is disposed on oneside of the slow wave circuit with its longitudinal axis parallel to theelectron beam path. Second lossy transmission line 60 is similarlyoriented on the other side of the slow wave circuit. Transmission lines56 and 60 are coupled into the interaction cavity by means of couplingiries 63 and 64, respectively. As shown each transmission line may behollow and with a center hole (50 and 52 respectively) though which alossy fluid is passed to remove heat and provide a high rf powerabsorption capability.

The lossy transmission lines may, of course, be of other constructionsand materials. Further details of other transmission lines and otherprior art arrangements may be found in the afore-mentioned U.S. Pat. No.3,324,338 and U.S. Pat. No. 3,771,010.

Referring to FIGS. 3A and 3B a particular embodiment of the presentinvention is shown. Coupling irises 65 and 66 in FIG. 3A couple thelossy transmission lines (56 and 60) directly to coupling hole 48. InFIG. 3B the next or alternate spacer element 42 has its coupling hole 49directly coupled to the lossy transmission lines (56 and 60) by couplingirises 67 and 68, respectively.

A second specific embodiment changing the angle of coupling is shown inFIGS. 4A and 48. FIG. 4A shows the first spacer element 42 with itscoupling hole (48) directly coupled by iris 69 only to transmission line60 and not directly coupled to transmission line 56. FIG. 4B showsalternate spacer element 42' with its coupling hole (49) directlycoupled by coupling iris 72 only to transmission line 56. Other anglesmay be chosen than the particular set shown in FIG. 4A and FIG. 4B. Theconfiguration of FIG. 3A and FIG. 38 represents a zero(normal)anglearrangement.

Two other aspects of the invention may be discussed either references toFIG. 3A. The first dimension of interest is the distance w between liney-y and line z-z. The line y-y' is a line tangent to the edge of theinteraction cavity. Distance w may be varied by moving the perimeter oflossy transmission line 56 (line z-z') closer to line y-y or past liney-y (w) into the edge of the interaction cavity. The loss increases as wdecreases to zero or goes negative. This position may of course bevaried on either or both lines (56 and 60).

Loss may also be varied by varing the coupling iris (66) width.

To attain high electronic efficiency and stability the degree of lossper unit length must be easily varied. The embodiment and changesdiscussed above allow a particular desired degree of loss to be attainedin a simple manner. As above discussed, a basic change over the priorart is coupling the transmission lines directly to the coupling holesvia the coupling irises. Further changes in loss are attained by theposition of the transmission lines with respect to the interactioncavity, width of the irises, and angel of the irises.

The particular degress of loss desired is in the range of 0.3 d 2.0.Here d is the normalized Pierce loss parameter (i.e. normalizeddbs/wavelength). Different loss levels are required by different tubestructures and different points in a particular tube may also requiredifferent loss levels. Since the loss (in db) from the coupled line hasthe greatest affect on the d parameter, it is necessary to obtain theparticular d level desired.

For example, in a particular structure at l 1 Ghz and angle zero (FIGS.3A and 3B) doubling the slot width gave a change in loss from 0.08 to1.21 (.db/cavity). Another test of a FIG. 3A structure at l 1 Ghz showedloss equal to 1.37 (db/cavity) with w equal to zero (tangent) and lossequal to 4.14 (db/cavity) with w equal to .05 inch (or 0.05 into theinteraction cavity). Thus it is seen that coupling the transmission lineor lines directly to the coupling holes in the various arrangementsdiscussed above allows the loss and thus d to be easily and widelyvaried as desired.

Obviously many modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed and desired to be secured by Letters Patent of theUnited States is:

1. In a traveling-wave tube having means for providing a stream ofelectrons along a predetermined path and means propagatingelectromagnetic energy comprising means defining a plurality ofintercoupled interaction cavities in a manner so that interaction takesplace with the stream of electrons further comprising:

a plurality of support pole pieces, each having at least one couplinghole means providing intercoupling of electromagnetic energy betweensaid interaction cavities;

at least one lossy transmission line means disposed proximate andexternal to said predetermined path;

and

means for preventing bondedge oscillation comprising a plurality ofcoupling irises located in said support pole pieces coupling directly atleast one of said lossy transmission line means and at least one of saidcoupling means.

2. The traveling-wave tube of claim 1, wherein:

said transmission line means is tangent to said interaction cavities.

3. The traveling-wave tube of claim 1, wherein:

said transmission line means is external of said interaction cavities.

4. The traveling-wave tube of claim 1, wherein:

said transmission line means is at least partially disposed in saidinteraction cavities.

5. The traveling-wave tube of claim 1, wherein:

' said transmission lines are hollow tubes adapted for lossy fluid flowtherein. 9} The traveling-wave tube of claim 6, wherein: saidtransmission line means comprises two transmission lines; and saidcoupling irises couple both of said transmission lines to each alternatecoupling hole means. 10. The traveling-wave tube of claim 9, wherein:said transmission lines are hollow tubes adapted for

1. In a traveling-wave tube having means for providing a stream ofelectrons along a predetermined path and means propagatingelectromagnetic energy comprising means defining a plurality ofintercoupled interaction cavities in a manner so that interaction takesplace with the stream of electrons further comprising: a plurality ofsupport pole pieces, each having at least one coupling hole meansproviding intercoupling of electromagnetic energy between saidinteraction cavities; at least one lossy transmission line meansdisposed proximate and external to said predetermined path; and meansfor preventing bondedge oscillation comprising a plurality of couplingirises located in said support pole pieces coupling directly at leastone of said lossy transmission line means and at least one of saidcoupling means.
 2. The traveling-wave tube of claim 1, wherein: saidtransmission line means is tangent to said interaction cavities.
 3. Thetraveling-wave tube of claim 1, wherein: said transmission line means isexternal of said interaction cavities.
 4. The traveling-wave tube ofclaim 1, wherein: said transmission line means is at least partiallydisposed in said interaction cavities.
 5. The traveling-wave tube ofclaim 1, wherein: the width of said coupling irises is in the range ofone half to equal the outside diameter of said transmission line means.6. The traveling-wave tube of claim 1, wherein: said transmission linemeans comprises two transmission lines; and said coupling irises couplesaid transmission lines alternatively to alternative coupling holemeans.
 7. The traveling-wave tube of claim 6, wherein: said couplingirises are disposed between 0* and 90* of the normal plane.
 8. Thetraveling-wave tube of claim 6, wherein: said transmission lines arehollow tubes adapted for lossy fluid flow therein.
 9. The traveling-wavetube of claim 6, wherein: said transmission line means comprises twotransmission lines; and said coupling irises couple both of saidtransmission lines to each alternate coupling hole means.
 10. Thetraveling-wave tube of claim 9, wherein: said transmission lines arehollow tubes adapted for lossy fluid flow therein.